Acidic treatment fluids comprising scleroglucan and/or diutan and associated methods

ABSTRACT

Treatment fluids are provided that comprise a gelling agent that comprises scleroglucan and/or diutan.

CROSS-REFERENCE TO A RELATED APPLICATION

This application is related to, application Ser. No. 11/118,028,currently pending, filed on Apr. 29, 2005.

BACKGROUND

The present invention relates to acidic treatment fluids used inindustrial and oil field operations, and more particularly, to acidictreatment fluids comprising gelling agents that comprise scleroglucanand/or diutan, and their use in industrial and oil field operations. Asused herein, the term “treatment fluid” refers to any fluid that may beused in a subterranean application in conjunction with a desiredfunction and/or for a desired purpose. The term “treatment fluid” doesnot imply any particular action by the fluid or any component thereof.

Acidizing and fracturing procedures using acidic treatment fluids arecommonly carried out in subterranean well formations to accomplish anumber of purposes including, but not limited to, to facilitate therecovery of desirable hydrocarbons from the formation. One commonly usedaqueous acidic treatment fluid comprises hydrochloric acid. Othercommonly used acids for acidic treatment fluids include: hydrofluoricacid, acetic acid, formic acid, citric acid, ethylene diamine tetraacetic acid (“EDTA”), glycolic acid, sulfamic acid, and derivatives orcombinations thereof.

Acidic treatment fluids are used in various subterranean operations. Forexample, formation acidizing or “acidizing” is a well known method forincreasing the flow of desirable hydrocarbons from a subterraneanformation. In a matrix acidizing procedure, an aqueous acidic treatmentfluid is introduced into a subterranean formation via a well boretherein under pressure so that the acidic treatment fluid flows into thepore spaces of the formation and reacts with the acid-soluble materialstherein. As a result, the pore spaces of that portion of the formationare enlarged, and consequently, the permeability of the formation shouldincrease. The flow of hydrocarbons from the formation is thereforeincreased because of the increase in formation conductivity caused,inter alia, by dissolution of the formation material. In fractureacidizing procedures, one or more fractures are produced in theformations and an acidic treatment fluid is introduced into thefracture(s) to etch flow channels therein. Acidic treatment fluids alsomay be used to clean out well bores to facilitate the flow of desirablehydrocarbons. Other acidic treatment fluids may be used in diversionprocesses, and well bore clean-out processes. A specific example isfilter cake removal.

To increase the viscosity of an aqueous acid treatment fluid, a suitablegelling agent may be included in the treatment fluid (often referred toas “gelling” the fluid). Gelling an aqueous acidic treatment fluid maybe useful to prevent the acid from becoming prematurely spent andinactive. Additionally, gelling an aqueous acidic treatment fluid mayenable the development of wider fractures so that live acid may beforced further into the formation from the well bore. Gelling the acidictreatment fluid may delay the interaction of the acid with an acidsoluble component in the well bore or the formation. Moreover, gellingan aqueous acidic treatment fluid may permit better fluid loss controlof the fluid.

Acidic treatment fluids used in subterranean operations arepredominantly water-based fluids that comprise gelling agents that mayincrease their viscosities, inter alia, to provide viscosity to controlthe rate of spending of the acid. These gelling agents are usuallybiopolymers or synthetic polymers that, when hydrated and at asufficient concentration, are capable of forming a more viscous fluid.Common gelling agents include polysaccharides (such as xanthan),synthetic polymers (such as polyacrylamide), and surfactant gel systems.Acidic treatment fluids comprising xanthan generally have sufficientviscosity for lower temperature operations. At elevated temperatures(e.g., those above about 120° F. to about 150° F.), however, theviscosity of such xanthan treatment fluids are diminished. Consequently,xanthan may not be a suitable gelling agent for acidic treatment fluidswhen those fluids are used in well bores that comprise elevatedtemperatures. Other gelling agents such as synthetic gelling agents(e.g., polyacrylamides) have been used, but they are often difficult todisperse and usually require considerable mixing or agitation to developfull viscosity. Additionally, most conventional gelling agents,including guar and some synthetic polymers, may form acid insolubleresidues. Moreover, surfactant gel systems can be expensive, and areoften sensitive to impurities. Also, surfactant gel systems oftenrequire hydrocarbon breakers.

SUMMARY OF THE INVENTION

The present invention relates to acidic treatment fluids used inindustrial and oil field operations, and more particularly, to acidictreatment fluids comprising gelling agents that comprise scleroglucanand/or diutan, and their use in industrial and oil field operations.

In another embodiment, the present invention provides a subterraneanwell treatment system comprising an acidic treatment fluid thatcomprises an aqueous base fluid, an acid, and a gelling agent thatcomprising scleroglucan and/or diutan.

In another embodiment, the present invention provides an acidictreatment fluid comprising a gelling agent that comprises scleroglucanand/or diutan.

The features and advantages of the present invention will be readilyapparent to those skilled in the art upon a reading of the descriptionof the preferred embodiments that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present invention, and should not be used to limit or define theinvention.

FIG. 1 illustrates viscosity data from an experiment involving anembodiment of the present invention.

FIG. 2 illustrates the viscosity data from an experiment involving anembodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to acidic treatment fluids used inindustrial and oil field operations, and more particularly, to acidictreatment fluids comprising gelling agents that comprise scleroglucanand/or diutan, and their use in industrial and oil field operations.Such operations may involve the removal of scale, fracture acidizing,matrix acidizing, diversion, filter cake removal, or pill removal.

In certain embodiments, the present invention provides fluids andmethods that are especially suitable for use in well bores comprising aborehole temperature (“BHT”) of up to about 500° F. A preferredtemperature range is a treating temperature below about 250° F. Oneshould note that the ability of the acidic treatment fluids of thepresent invention to maintain a degree of viscosity at such elevatedtemperatures may be affected by the time a particular fluid is exposedto such temperatures. For example, in some fracture acidizingapplications, there may be a considerable fracture cool-down, which mayenable utilization of an acidic treatment fluid of the present inventionat BHT above the temperature limit at which the fluid demonstratesviscosity. One of the many advantages of the gelling agents of thepresent invention is that they typically do not leave undesirableresidues in the formation once the fluid has been broken. Anotheradvantage is that the gelling agents are environmentally acceptable insome sensitive environments (such as the North Sea). Additionally, thegelling agents of the present invention may present a cost savings oversome conventional gelling agents (like many surfactant-based gellingagents) for acidic treatment fluid applications. The acidic treatmentfluids of the present invention may be useful in a wide variety ofsubterranean treatment operations in which acidic treatment fluids maybe suitable.

The acidic treatment fluids of the present invention generally comprisean aqueous base fluid, an acid, and a gelling agent of the presentinvention that comprises scleroglucan and/or diutan. When used indiversion applications, the treatment fluid may or may not comprise anacid. One of ordinary skill in the art with the benefit of thisdisclosure will be able to determine whether an acid is appropriate.Generally speaking, the fluids of the present invention have a pH ofless than about 4. In preferred embodiments comprising hydrochloricacid, the treatment fluids may have a pH of about 1 or less. Inembodiments comprising an organic acid, the treatment fluids may have apH of about 1 to about 4.

The aqueous base fluids of the treatment fluids of the present inventiongenerally comprise fresh water, salt water, or a brine (e.g., asaturated salt water). Other water sources may be used, including thosecomprising divalent or trivalent cations, e.g., magnesium, calcium,zinc, or iron. Monovalent brines are preferred and, where used, may beof any weight. One skilled in the art will readily recognize that anaqueous base fluid containing a high level of multi-valent ions shouldbe tested for compatibility prior to use. Salts optionally may be addedto the water source, inter alia, to produce a treatment fluid having adesired density or other characteristics. One of ordinary skill in theart with the benefit of this disclosure will recognize the particulartype of salt appropriate for particular application, givenconsiderations such as protection of the formation, the presence orabsence of reactive clays in the formation adjacent to the well bore,compatibility with the other acidic treatment fluid additives, and thefactors affecting wellhead control. A wide variety of salts may besuitable. Examples of suitable salts include, inter alia, potassiumchloride, sodium bromide, ammonium chloride, cesium formate, potassiumformate, sodium formate, sodium nitrate, calcium bromide, zinc bromide,and sodium chloride. A preferred aqueous base fluid is a 5% ammoniumchloride brine with hydrofluoric acid or an organic acid. An artisan ofordinary skill with the benefit of this disclosure will recognize theappropriate concentration of a particular salt to achieve a desireddensity given factors such as the environmental regulations that maypertain. Also, the composition of the water used also will dictatewhether and what type of salt is appropriate. The amount of the basefluid in an acidic treatment fluid of the present invention will varydepending on the purpose of the fluid, the formation characteristics,and whether the fluid will be foamed.

Suitable acids for inclusion in the treatment fluids of the presentinvention include any acid suitable for use in a subterraneanapplication. Examples include hydrochloric acid, hydrofluoric acid,acetic acid, formic acid, citric acid, ethylene diamine tetra aceticacid (“EDTA”), glycolic acid, sulfamic acid, and derivatives or acombination thereof. Hydrochloric acid, acetic acid, or formic acid maybe preferred in certain applications. One should note that the choice ofaqueous base fluid and acid should be chosen vis-à-vis the other so thatthe proper synergistic effect is achieved. The concentration and type ofacid selected may be based upon the function of the acid (e.g., scaleremoval, fracture acidizing, matrix acidizing, removal of fluid lossfilter cakes and pills, and the like) and the mineralogy of theformation. It is well known that certain concentrations of acids willform precipitates upon spending. See Gdanski, R. D.: “Kinetics of theTertiary Reaction of HF on Alumino-Silicates”, SPE 31076 presented atthe SPE Formation Damage Symposium, Lafayette, La., Feb. 14-15, 1996.Such tendency to form precipitates should be taken into considerationwhen choosing an acid. A precipitation control additive (e.g., aluminumchloride) may be desirable to include as well depending on the acid andthe formation.

The gelling agents of the present invention may comprise scleroglucanand/or diutan. The gelling agent may be present in an acidic treatmentfluid of the present invention in an amount of from about 10 lb/Mgal toabout 200 lb/Mgal. Generally speaking, an acidic treatment fluidcontaining an organic acid may require less of a gelling agent of thepresent invention than an acidic treatment fluid containing hydrochloricacid.

As noted in the text BIOPOLYMERS, VOLUME 6, POLYSACCHARIDES II:POLYSACCHARIDES FROM EUKARYOTES, by E. J. Vandamme (Editor), S. De Baets(Editor), Alexander Steinbüchel (Editor), ISBN: 3-527-30227-1; publishedby Wiley 2002, specifically Chapters 2 and 3, scleroglucan is a neutralfungal polysaccharide. Scleroglucan is a hydrophilic polymer, which isbelieved to have a tendency to thicken and stabilize water-based systemsby conferring on them a relatively high viscosity, generally higher thanthat obtained in the case of xanthan, for example, at temperatures at orabove about 200° F., for identical concentrations of active compounds.Scleroglucan also appears to be more resistant to pH and temperaturechanges than xanthan, and therefore, may impart more stable viscosity insuch conditions. In certain aspects, the viscosity of a scleroglucanfluid may be virtually independent of pH between a pH of about 1 andabout 12.5 up to a temperature limit of about 270° F. Generally, themain backbone polymer chain of scleroglucan comprises(1→3)β-D-glucopyranosyl units with a single β-D-glucopyranosyl groupattached to every third unit on the backbone. Scleroglucan is thought tobe resistant to degradation, even at high temperatures such as those ator above about 200° F., even after, e.g., 500 days in seawater.Viscosity data (see Table 1 and Table 2) show that dilute solutions(e.g., about 0.5%) may be shear thinning and stable to at least 250° F.Note that these solutions are not acidic. These viscosities illustrate,inter alia, scleroglucan's suitability for viscosifying fluids. Inembodiments wherein the gelling agent of the present invention comprisesscleroglucan, one may include about 10 to about 200 lb/Mgalscleroglucan. In an acidic treatment fluid that comprises hydrochloricacid, a more preferred range may be from about 40 to about 120 lb/Mgalof scleroglucan.

TABLE 1 Viscosities (cP) of 1% Scleroglucan, Measured at VariousTemperatures (° C.) and Shear Rates (s⁻¹), using a Brookfield PVSRheometer Shear Rate (s⁻¹) 70° C. 80° C. 99° C. 108° C. 118° C. 127° C.8.5 1500 1450 1480 1460 1330 1540 25 520 540 540 550 500 — 85 180 180178 175 165 — 170 100 98 99 93 92 —

TABLE 2 Elastic Moduli G′ (Pa) Measured Using a Haake RS 150 ControlledStress Rheometer at 25° C.; Measurements Made at 1 Hz in the LinearViscoelastic Region. Xanthan Scleroglucan 1.0% 38 35 0.5% 9 13

As noted in the text BIOPOLYMERS, VOLUME 6, POLYSACCHARIDES II:POLYSACCHARIDES FROM EUKARYOTES, by E. J. Vandamme (Editor), S. De Baets(Editor), Alexander Steinbüchel (Editor), ISBN: 3-527-30227-1; publishedby Wiley 2002, specifically Chapters 2 and 3, and BIOPOLYMERS; (1999)vol 50; p. 496; Authors: B. H. Falch; A. Elgsaeter & B. T. Stokke,diutan gum is a polysaccharide designated as “S-657,” which is preparedby fermentation of a strain of sphingomonas. Diutan's structure has beenelucidated as a hexasaccharide having a tetrasaccharide repeat unit inthe backbone that comprises glucose and rhamnose units and di-rhamnoseside chain. It is believed to have thickening, suspending, andstabilizing properties in aqueous solutions. Diutan is composedprincipally of carbohydrates, about 12% protein, and about 7%(calculated as O-acetyl) acyl groups, the carbohydrate portioncontaining about 19% glucuronic acid, and the neutral sugars rhamnoseand glucose in the approximate molar ratio of about 2:1. Details of thediutan gum structure may be found in an article by Diltz et al.,“Location of O-acetyl Groups in S-657 Using the Reductive-CleavageMethod,” CARBOHYDRATE RESEARCH, Vol. 331, p. 265-270 (2001), which ishereby incorporated by reference in its entirety. Details of preparingdiutan gum may be found in U.S. Pat. No. 5,175,278, which is herebyincorporated by reference in its entirety. A suitable source of diutanis “GEOVIS XT,” which is commercially available from Kelco Oil FieldGroup, Houston, Tex. The elastic moduli of some diutan solutions ascompared to xanthan solutions are shown in Table 3. Note that these arenot acidic solutions. In embodiments wherein the gelling agent of thepresent invention comprises diutan, one may include about 10 to about200 lb/Mgal scleroglucan. In an acidic treatment fluid that comprisesabout 15% hydrochloric acid, a more preferred range may be from about100 to about 200 lb/Mgal of diutan.

TABLE 3 Elastic Moduli (G′) of Diutan and Xanthan Solutions SolutionComposition G′(Pa)  0.5% Diutan in water 15.0  0.5% Xanthan in water11.8  0.5% Diutan in 6% NaCl 19.0  0.5% Xanthan in 6% NaCl 12.8 0.75%Diutan in water 33.0 0.75% Diutan in 20% KCl 29.0

In some embodiments, the gelling agents may be at least partiallycrosslinked through a crosslinking reaction comprising a suitablecrosslinking agent. Suitable crosslinking agents include zirconium-basedcrosslinking agents, chrome-based crosslinking agents, and iron-basedcrosslinking agents. Crosslinking the gelling agent may be desirablewhere it is desirable to make a certain acidic treatment fluid moreviscous. One of ordinary skill in the art with the benefit of thisdisclosure will recognize when such crosslinkers are appropriate andwhat particular crosslinker will be most suitable. Things to take intoconsideration when choosing a suitable crosslinking agent include the pHrange of the fluid, activity of the crosslinking agent, the desiredviscosity of the treatment fluid, the temperature sensitivity of thecrosslinking agent, and the sheer sensitivity of the fluid in theenvironment. It should be noted that suitable viscosities could beobtained for acidic treatment fluids that comprise gelling agents thatcomprise diutan without using crosslinkers. Typically, a crosslinkingagent may be included in an amount of from about 0.01 lb/Mgal to about15 lb/Mgal.

Typical cross-linking agents are transitional metals and/or transitionmetal complexes such as iron, titanium, chromium and zirconium includingreaction products of organic acids including polyfunctional acidsincluding dicarboxylic acids, hydroxy-carboxylic acids, amine-carboxylicacids (including for example acetic acid, oxalic acid, malonic acid,succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid,azelaic acid, sebacic acid, maleic acid, fumaric acid, lactic acid,aspartic acid, malic acid, mandelic acid, citric acid, and the like).Particularly useful are the hydroxy-carboxylic acids such as lactic,maleic and citric acids. Also useful are the complexes formed with thesecompounds and ammonia alkyli metals, including methyl amine, propylamine, diethylamine, triethylene tetramine, isopropyl amine, and thelike; and hydroxylamines such as triethanolamine, diethanol amine, andthe like. Typical compounds include ferric chloride, titanium lactate,titanium malate, titanium citrate, zirconium lactate, zirconiumoxychloride, zirconium hydroxychloride, zirconium citrate, zirconiumcomplex of hydroxyethyl glycine, ammonium zirconium fluoride, zirconium2-ethylhexanoate, zirconium acetate, zirconium neodecanoate, zirconiumacetylacetonate, tetrakis(triethanolamine)zirconate, zirconiumcarbonate, ammonium zirconium carbonate, zirconyl ammonium carbonate,zirconium lactate, titanium acetylacetonate, titanium ethylacetoacetate,titanium citrate, titanium triethanolamine, ammonium titanium lactate,aluminum citrate, chromium citrate, chromium acetate, chromiumpropionate, chromium malonate, zirconium malate, ammonium, sodiumzirconium lactate, zirconium lactate in combination with isopropylamineor triethanolamine, mixtures thereof and the like. Also useful is theuse of crosslinking retarders include tartaric acid, sodiumglucoheptonate, glucono-delta lactone, sodium lignosulfonate,combinations there of, and the like.

In certain embodiments, the acidic treatment fluids of the presentinvention also may comprise suitable: hydrate inhibitor, corrosioninhibitors, pH control additives, surfactants, breakers, fluid losscontrol additives, scale inhibitors, asphaltene inhibitors, paraffininhibitors, salts, foamers, defoamers, emulsifiers, demulsifiers, ironcontrol agents, solvents, mutual solvents, particulate diverters, gasphase, carbon dioxide, nitrogen, other biopolymers, synthetic polymers,friction reducers combinations thereof, or the like. The acidictreatment fluids of the present invention also may include otheradditives that may be suitable for a given application.

In alternative embodiments, the acidic treatment fluids of the presentinvention may be foamed. In such embodiments, the acidic treatmentfluids also comprise a gas, and a foaming agent. While various gases canbe utilized for foaming the acidic treatment fluids of this invention,nitrogen, carbon dioxide, and mixtures thereof are preferred. Inexamples of such embodiments, the gas may be present in an acidictreatment fluid in an amount in the range of from about 5% to about 95%by volume of the treatment fluid, and more preferably in the range offrom about 20% to about 80%. The amount of gas to incorporate into thefluid may be affected by factors including the viscosity of the fluidand wellhead pressures involved in a particular application. Examples ofpreferred foaming agents that can be utilized to foam and stabilize theacidic treatment fluids of this invention include, but are not limitedto, alkylamidobetaines such as cocoamidopropyl betaine, alpha-olefinsulfonate, trimethyltallowammonium chloride, C₈ to C₂₂ alkylethoxylatesulfate and trimethylcocoammonium chloride. Cocoamidopropyl betaine isespecially preferred. Other suitable surfactants available fromHalliburton Energy Services include: “19N,” “G-Sperse Dispersant,”“Morflo III®” surfactant, “Hyflo (R) IV M” surfactant, “Pen-88M™”surfactant, “HC-2 Agent,” “Pen-88 HT™” surfactant, “SEM-7™” emulsifier,“Howco-Suds™” foaming agent, “Howco Sticks™” surfactant, “A-Sperse™”Dispersing aid for acid additives, “SSO-21E” surfactant, and “SSO-21MW™”surfactant. Other suitable foaming agents and foam stabilizing agentsmay be included as well, which will be known to those skilled in the artwith the benefit of this disclosure. The foaming agent is generallypresent in an acidic treatment fluid of the present invention in anamount in the range of from about 0.1% to about 2.0% by weight, morepreferably in the amount of from about 0.2% to about 1.0% and mostpreferably about 0.6%.

Examples of suitable corrosion inhibitors include acetylenic alcohols,Mannich condensation products (such as those formed by reacting analdehyde, a carbonyl containing compound and a nitrogen containingcompound), unsaturated carbonyl compounds, unsaturated ether compounds,formamide, formic acid, formates, other sources of carbonyl, iodides,terpenes, and aromatic hydrocarbons, coffee, tobacco, gelatin,cinnamaldehyde, cinnamaldehyde derivatives, acetylenic alcohols,fluorinated surfactants, quaternary derivatives of heterocyclic nitrogenbases, quaternary derivatives of halomethylated aromatic compounds,formamides, combinations of such compounds used in conjunction withiodine; quaternary ammonium compounds; and combinations thereof.Suitable corrosion inhibitors and intensifiers are available fromHalliburton Energy Services and include: “MSA-II™” corrosion inhibitor,“MSA-III” corrosion inhibitor, “HAI-25 E+” environmentally friendly lowtemp corrosion inhibitor, “HAI-404™” acid corrosion inhibitor, “HAI-50™”Inhibitor, “HAI-60™” Corrosion inhibitor, “HAI-62™” acid corrosioninhibitor, “HAI-65™” Corrosion inhibitor, “HAI-72E+™” Corrosioninhibitor, “HAI-75™” High temperature acid inhibitor, “HAI-81M™” Acidcorrosion inhibitor, “HAI-85™” Acid corrosion inhibitor, “HAI-85M™” Acidcorrosion inhibitor, “HAI-202 Environmental Corrosion Inhibitor,”“HAI-OS” Corrosion Inhibitor, “HAI-GE” Corrosion Inhibitor,“FDP-S692-03” Corrosion inhibitor for organic acids, “FDP-S656AM-02” and“FDP-S656BW-02” Environmental Corrosion Inhibitor System, “HII-500”Corrosion inhibitor intensifier, “HII-500M” Corrosion inhibitorintensifier, “HII-124” Acid inhibitor intensifier, “HII-124B” Acidinhibitor intensifier, “HII-124C™” Inhibitor intensifier, and“HII-124F™” corrosion inhibitor intensifier. Suitable iron controlagents are available from Halliburton Energy Services and include:“Fe-2™” Iron sequestering agent, “FE-2A™” Buffering agent, “FE-3™” Ironcontrol agent, “FE-3A™” Iron control agent, “FE-4™” Iron control agent,“FE-5™” Iron control agent, “FE-5A™” Iron control agent, “FERCHEK®”Ferric iron inhibitor, “FERCHEK®” A “Reducing agent, and “FERCHEK® SC”Iron control process or system. Other suitable iron control agentsinclude those described in U.S. Pat. Nos. 6,315,045, 6,525,011,6,534,448, and 6,706,668. Examples of corrosion inhibitor activatorsthat may be included include, but are not limited to, cuprous iodide;cuprous chloride; antimony compounds such as antimony oxides, antimonyhalides, antimony tartrate, antimony citrate, alkali metal salts ofantimony tartrate and antimony citrate, alkali metal salts ofpyroantimonate and antimony adducts of ethylene glycol; bismuthcompounds such as bismuth oxides, bismuth halides, bismuth tartrate,bismuth citrate, alkali metal salts of bismuth tartrate and bismuthcitrate; iodine; iodide compounds; formic acid; and mixtures of theforegoing activators such as a mixture of formic acid and potassiumiodide. The amount of any corrosion inhibitor to include in an acidictreatment fluid of the present invention will depend on many factors,including but not limited to, the metallurgy the acid will contact,contact time, temperature, etc. Generally, the amount of a corrosioninhibitor to include will range from about 0.1% to about 3% by volume.

Suitable pH control additives, in certain embodiments, may comprisebases, chelating agents, acids, or combinations of chelating agents andacids or bases. A pH control additive may be necessary to maintain thepH of the treatment fluid at a desired level, e.g., to improve thedispersion of the gelling agent in the aqueous base fluid. In someinstances, it may be beneficial to maintain the below 3. Suitable pHcontrol additives are those additives that assist in maintaining the pHof an acidic treatment fluid very low, and may include glycolic acids,acetic acids, lactone derivatives, formic acid, carbonic acid, sulfamicacid, and the like.

In some embodiments, the acidic treatment fluids of the presentinvention may include surfactants, e.g., to improve the compatibility ofthe acidic treatment fluids with other fluids (like any formationfluids) that may be present in the well bore. Examples of suitablesurfactants include ethoxylated nonyl phenol phosphate esters, nonionicsurfactants, cationic surfactants, anionic surfactants, amphotericsurfactants, alkyl phosphonate surfactants, linear alcohols, nonylphenolcompounds, alkyoxylated fatty acids, alkylphenol alkoxylates,ethoxylated amides, ethoxylated alkyl amines, amphoteric surfactants(such as betaines), and mixtures thereof. Suitable surfactants may beused in a liquid or powder form. In an embodiment where liquidsurfactants are used, the surfactants are generally present in an amountin the range of from about 0.01% to about 5.0% by volume of the acidictreatment fluid. In one embodiment, the liquid surfactants are presentin an amount in the range of from about 0.1% to about 2.0% by volume ofthe acidic treatment fluid. In embodiments where powdered surfactantsare used, the surfactants may be present in an amount in the range offrom about 0.001% to about 0.5% by weight of the acidic treatment fluid.Examples of suitable surfactants are non-emulsifiers commerciallyavailable from Halliburton Energy Services, Inc., of Duncan, Okla.,under the tradenames “LOSURF-259™” solid surfactant, “LOSURF-300™”nonionic surfactant, “LOSURF-357™” nonionic surfactant, and“LOSURF-400™,” surfactant, “LOSURF-2000S™” solid surfactant, and“LOSURF-2000M” solid surfactant, “LOSURF-357” nonionic surfactant,“LOSURF-400” surfactant, “LOSURF-2000S” surfactant, “LOSURF-259”nonionic non-emulsifier, and “LOSURF-300” nonionic surfactant. Anotherexample of a suitable surfactant is a non-emulsifier commerciallyavailable from Halliburton Energy Services, Inc., of Duncan, Okla.,under the tradename “NEA-96M™” Surfactant. Other examples of suitablesurfactants that are commercially available from Halliburton EnergyServices in Duncan, Okla. are trade named products “SGA-1,” “EFS-1,”“EFS-2,” “EFS-3,” and “EFS-4.”

While typically not required, the acidic treatment fluids of the presentinvention also may comprise breakers capable of reducing the viscosityof the acidic treatment fluid at a desired time. Examples of suchsuitable breakers for acidic treatment fluids of the present inventioninclude, but are not limited to, sodium chlorite, hypochlorite,perborate, persulfates, peroxides, including organic peroxides. Othersuitable breakers include suitable acids. Preferred examples of suitablebreakers for acidic treatment fluids of the present invention thatinclude a gelling agent that comprises diutan include peroxide breakers.Preferred examples include tert-butyl hydroperoxide and tert-amylhydroperoxide. Sodium persulfate and sodium chlorite are not preferredbreakers for acidic treatment fluids of the present invention thatinclude a gelling agent that comprises diutan because optimaldegradation generally may not occur within a desirable time period. Abreaker may be included in an acidic treatment fluid of the presentinvention in an amount and form sufficient to achieve the desiredviscosity reduction at a desired time. The breaker may be formulated toprovide a delayed break, if desired. For example, a suitable breaker maybe encapsulated if desired. Suitable encapsulation methods are known tothose skilled in the art. One suitable encapsulation method that may beused involves coating the chosen breakers with a material that willdegrade when downhole so as to release the breaker when desired. Resinsthat may be suitable include, but are not limited to, polymericmaterials that will degrade when downhole. The terms “degrade,”“degradation,” or “degradable” refer to both the two relatively extremecases of hydrolytic degradation that the degradable material mayundergo, i.e., heterogeneous (or bulk erosion) and homogeneous (orsurface erosion), and any stage of degradation in between these two.This degradation can be a result of, inter alia, a chemical or thermalreaction or a reaction induced by radiation. Suitable examples ofmaterials that can undergo such degradation include polysaccharides suchas dextran or cellulose; chitins; chitosans; proteins; aliphaticpolyesters; poly(lactides); poly(glycolides); poly(ε-caprolactones);poly(hydroxybutyrates); poly(anhydrides); aliphatic polycarbonates;orthoesters, poly(orthoesters); poly(amino acids); poly(ethyleneoxides); and polyphosphazenes. If used, a breaker should be included ina composition of the present invention in an amount sufficient tofacilitate the desired reduction in viscosity in a viscosifier treatmentfluid. For instance, peroxide concentrations that may be used vary fromabout 0.1 to about 10 gallons of peroxide per 1000 gallons of the acidictreatment fluid. Optionally, the acidic treatment fluid may contain anactivator or a retarder, inter alia, to optimize the break rate providedby the breaker.

To facilitate a better understanding of the present invention, thefollowing examples of certain aspects of some embodiments are given. Inno way should the following examples be read to limit, or define, thescope of the invention.

EXAMPLES

Fluid preparation: Acidic treatment fluids comprising scleroglucan wereprepared by making a 15% HCl fluid containing 83.5 lb/Mgal ofscleroglucan in a Waring blender. The fluid was mixed (hydrated) for 30minutes. Acidic treatment fluids comprising a diutan gelling agent and axanthan gelling agent were prepared in a similar manner using 164.9lb/Mgal and 83.5 lb/Mgal, respectively. A comparative xanthan fluid wasprepared using the same process.

Fluid evaluation: The fluids were then evaluated under two differenttemperature profiles on a Nordman Model 50 viscometer using a modifiedAPI2 test. The results are shown in FIG. 1 and FIG. 2. In FIG. 1, theapparent viscosity is given on the major Y-axis and the sampletemperature is given on the minor Y-axis. FIG. 1 demonstrates thatfluids comprising a gelling agent that comprises scleroglucan or diutancan maintain higher viscosities at higher temperatures than a fluidcomprising a gelling agent that comprises xanthan. Similarly, FIG. 2demonstrates that a fluid comprising a gelling agent that comprisesscleroglucan has a higher viscosity than a fluid that comprises agelling agent that comprises xanthan.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Whilenumerous changes may be made by those skilled in the art, such changesare encompassed within the spirit of this invention as defined by theappended claims. The terms in the claims have their plain, ordinarymeaning unless otherwise explicitly and clearly defined by the patentee.

1. A subterranean well treatment system comprising an acidic treatmentfluid that comprises an aqueous base fluid, an acid, and a gelling agentthat comprises diutan, wherein the pH of the acidic treatment fluid isless than about
 4. 2. The well treatment system of claim 1 wherein thegelling agent is at least partially crosslinked through a crosslinkingreaction that comprises a crosslinking agent.
 3. The well treatmentsystem of claim 2 wherein the crosslinking agent comprises at least oneof the following: an iron-based compound; a titanium-based compound; azirconium-based compound; ferric chloride; titanium lactate; titaniummalate; titanium citrate; zirconium lactate; zirconium oxychloride;zirconium hydroxychloride; zirconium citrate; a zirconium complex ofhydroxyethyl glycine; ammonium zirconium fluoride; zirconium2-ethylhexanoate; zirconium acetate; zirconium neodecanoate; zirconiumacetylacetonate; tetrakis(triethanolamine)zirconate; zirconiumcarbonate; animonium zirconium carbonate; zirconyl ammonium carbonate;zirconium lactate; titanium acetylacetonate; titanium ethylacetoacetate;titanium citrate; titanium triethanolamine; animonium titanium lactate;aluminum citrate; chromium citrate; chromium acetate; chromiumpropionate; chromium malonate; zirconium malate; ammonium; sodiumzirconium lactate; or zirconium lactate in combination withisopropylamine or triethanol amine.
 4. The well treatment system ofclaim 1 wherein the acidic treatment fluid comprises at least one of thefollowing: fresh water; salt water; a brine; a salt; potassium chloride;sodium bromide; ammonium chloride; cesium formate; potassium formate;sodium formate; sodium nitrate; calcium bromide; zinc bromide; sodiumchloride; hydrochloric acid; hydrofluoric acid; acetic acid; formicacid; citric acid; ethylene diamine tetra acetic acid; glycolic acid; orsulfamic acid.
 5. The well treatment system of claim 1 wherein thegelling agent is present in the acidic treatment fluid in an amount offrom about 10 lb/Mgal to about 200 lb/Mgal.
 6. The well treatment systemof claim 1 wherein the acidic treatment fluid comprises at least one ofthe following: a hydrate inhibitor; a corrosion inhibitor; a pH controladditive; a surfactant; a breaker; a fluid loss control additive; ascale inhibitor; an asphaltene inhibitor; a paraffin inhibitor; afoaming agent; a defoamer; an emulsifier; a demulsifier; an iron controlagent; a solvent; a mutual solvent; a particulate diverter; a gas;carbon dioxide; nitrogen; a biopolymer other than scleroglucan ordiutan; a synthetic polymer; or a friction reducer.
 7. The welltreatment system of claim 1 wherein the acidic treatment fluid comprisesa gas in the amount of from about 5% to about 95% by volume.
 8. The welltreatment system of claim 1 wherein the acidic treatment fluid comprisesat least one of the following: an alkylamidobetaine; cocoamidopropylbetaine; alpha-olefin sulfonate; trimethyltallowammonium chloride; a C₈to C₂₂ alkylethoxylate sulfate; or trimethylcocoammonium chloride. 9.The well treatment system of claim 1 wherein the acidic treatment fluidcomprises a corrosion inhibitor that comprises at least one of thefollowing: an acetylenic alcohol; a Mannich condensation product; anunsaturated carbonyl compound; an unsaturated ether compound; formamide;formic acid; a formate; a carbonyl; an iodide; a terpene; an aromatichydrocarbon; coffee; tobacco; gelatin; cinnamaldehyde; a cinnamaldehydederivative; an acetylenic alcohol; a fluorinated surfactant; a quatemaryderivative of heterocyclic nitrogen base; a quaternary derivative of ahalomethylated aromatic compound; cuprous iodide; cuprous chloride; anantimony compound; an antimony oxide; an antimony halide; antimonytartrate; antimony citrate; an alkali metal salt of an antimony tartrateand antimony citrate; an alkali metal salt of pyroantimonate and anantimony adduct of ethylene glycol; a bismuth compound; a bismuth oxide;a bismuth halide; a bismuth tartrate; a bismuth citrate; an alkali metalsalts of bismuth tartrate and bismuth citrate; or iodine.
 10. Atreatment fluid comprising a gelling agent that comprises diutan,wherein the pH of the treatment fluid is less than about
 4. 11. Thefluid of claim 10 wherein the gelling agent is at least partiallycrosslinked through a crosslinking reaction that comprises acrosslinking agent.
 12. The fluid of claim 11 wherein the crosslinkingagent comprises at least one of the following: an iron-based compound; atitanium-based compound; a zirconium-based compound; ferric chloride;titanium lactate; titanium malate; titanium citrate; zirconium lactate;zirconium oxychioride; zirconium hydroxychloride; zirconium citrate; azirconium complex of hydroxyethyl glycine; ammonium zirconium fluoride;zirconium 2-ethylhexanoate; zirconium acetate; zirconium neodecanoate;zirconium acetylacetonate; tetrakis(triethanolamine)zirconate; zirconiumcarbonate; ammonium zirconium carbonate; zirconyl ammonium carbonate;zirconium lactate; titanium acetylacetonate; titanium ethylacetoacetate;titanium citrate; titanium triethanolamine; ammonium titanium lactate;aluminum citrate; chromium citrate; chromium acetate; chromiumpropionate; chromium malonate; zirconium malate; ammonium; sodiumzirconium lactate; or zirconium lactate in combination withisopropylamine or triethanol amine.
 13. The fluid of claim 10 whereinthe acidic treatment fluid comprises at least one of the following:fresh water; salt water; a brine; a salt; potassium chloride; sodiumbromide; ammonium chloride; cesium formate; potassium formate; sodiumformate; sodium nitrate; calcium bromide; zinc bromide; sodium chloride;hydrochloric acid; hydrofluoric acid; acetic acid; formic acid; citricacid; ethylene diamine tetra acetic acid; glycolic acid; or sulfamicacid.
 14. The fluid of claim 10 wherein the gelling agent is present inthe acidic treatment fluid in an amount of from about 10 lb/Mgal toabout 200 lb/Mgal.
 15. The fluid of claim 10 wherein the acidictreatment fluid comprises at least one of the following: a hydrateinhibitor; a corrosion inhibitor; a pH control additive; a surfactant; abreaker; a fluid loss control additive; a scale inhibitor; an asphalteneinhibitor; a paraffin inhibitor; a foaming agent; a defoamer; anemulsifier; a demulsifier; an iron control agent; a solvent; a mutualsolvent; a particulate diverter; a gas; carbon dioxide; nitrogen; abiopolymer other than scleroglucan or diutan; a synthetic polymer; or afriction reducer.
 16. The fluid of claim 10 wherein the acidic treatmentfluid comprises a gas in the amount of from about 5% to about 95% byvolume.
 17. The fluid of claim 10 wherein the acidic treatment fluidcomprises at least one of the following: an alkylamidobetaine;cocoamidopropyl betaine; alpha-olefin sulfonate; trimethyltallowammoniumchloride; a C₈ to C₂₂ alkylethoxylate sulfate; or trimethylcocoammoniumchloride.
 18. The fluid of claim 10 wherein the acidic treatment fluidcomprises a corrosion inhibitor that comprises at least one of thefollowing: an acetylenic alcohol; a Mannich condensation product; anunsaturated carbonyl compound; an unsaturated ether compound; formamide;formic acid; a formate; a carbonyl; an iodide; a terpene; an aromatichydrocarbon; coffee; tobacco; gelatin; cinnamaldehyde; a cinnamaldehydederivative; an acetylenic alcohol; a fluorinated surfactant; a quatemaryderivative of heterocyclic nitrogen base; a quatemary derivative of ahalomethylated aromatic compound; cuprous iodide; cuprous chloride; anantimony compound; an antimony oxide; an antimony halide; antimonytartrate; antimony citrate; an alkali metal salt of an antimony tartrateand antimony citrate; an alkali metal salt of pyroantimonate and anantimony adduct of ethylene glycol; a bismuth compound; a bismuth oxide;a bismuth halide; a bismuth tartrate; a bismuth citrate; an alkali metalsalts of bismuth tartrate and bismuth citrate; or iodine.